Signal generators

ABSTRACT

A signal generator includes a piston-in-waveguide attenuator coupled to a tuned circuit, a control voltage whose magnitude depends on the position of the piston in the waveguide of the attenuator being produced from a potentiometer and applied to a variable capacity diode in the tuned circuit. Changes in the reactance of the tuned circuit caused by the proximity of the piston are compensated by changes in the capacity of the variable capacity diode, maintaining a constant generated frequency.

United States Patent McHattie June 6, 1972 SIGNAL GENERATORS ReferencesCited [72] Inventor: John Alexander McHattie, Dunstable, En- UNITEDSTATES PATENTS gland 2,701,863 2/1955 Pierce ..333/81 s [73] Assignee:Marconi Instruments Limited, London, 3,108,239 10/1963 Koueiter..307/320 X England Primary Examiner-Eli Lieberman [22] June I971Assistant Examiner-Marvin Nussbaum [21] Appl. No.: 157,309Attorney-Donald M. Wight et al.

[30] Foreign Application Priority Data [57] ABSTRACT July 30, 1970 GreatBritain ..36,872/7O US. Cl. ..333/8l B, 333/24 R, 333/82 B,

333/83 R, 334/15 Int. Cl ..H0lp 1/22, HOlp 7/06, H03j 3/06 Fieldot'Search ..334/l5;333/81 A,8l B, 24,

6 Claims, 1 Drawing Figure 20 18 Switch.

78 Switch.

INVENTOR 2% W ms/Jm ATTORNEYS SIGNAL GENERATORS This invention relatesto signal generators and more particularly to radio frequency signalgenerators of the kind employing a so-called piston-in-waveguideattenuator coupled to take output from a resonant circuit, e.g., aresonant cavity or line resonator forming part of the tuned output.circuit of the generator.

A piston-in-waveguide attenuator as used in signal generators consistsof a piston carrying pick-up element a pick-up resistor or an inductivepick-up loop mounted in a waveguide. One end of the waveguide isconnected to a resonant circuit which is part of the signal generatingcircuit. The piston is movable within the waveguide, and as the pistonis moved closer to the tuned circuit the pick-up element picks up asignal of increasing amplitude. Conversely as the piston is moved awayfrom the resonant circuit the pick-up element picks up a signal ofdecreasing amplitude. Thus the attenuation of the system is dependentupon the position of the piston in the waveguide.

A piston-in-waveguide attenuator suffers from the disadvantage that asthe piston approaches close to the resonant circuit, i.e., when theattenuation of the picked up signal is at a minimum, the proximity ofthe piston causes a changein the reactive loading of the resonantcircuit, hence causing a change in the frequency of oscillation. Thefrequency change, often known as attenuation reaction, is undesirableand a number of methods are in use to reduce the effect.

One way of reducing the effect is to mount a Faraday Screen of parallelwires across the mouth of the waveguide closest to the resonant circuit.The wires forming the screen must all lie in the same direction, or elsethe screen would form a short circuit and no electric waves would bepropagated in the output waveguide. However, by utilizing a screen ofparallel wires so that electric waves are still propagated in the outputwaveguide the screen is not completely effective and the proximity ofthe piston still affects the resonance frequency of the resonantcircuitalbeit to a lesser extent than if the Faraday Screen were not utilized.

An alternative method of reducing the effect is to utilize a bufferamplifier between the resonant circuit and the output waveguide. Thishas the disadvantage that the buffer amplifier is a further activeelement in the system, which may increase the noise level, as well asadding to the costs of manufacture. A further method is to limit thepossible travel of the piston in the waveguide so that it cannotapproach close enough to the resonant circuit to affect its reactiveloading and hence alter its resonance frequency. If this method is usedthe picked up signal will always be attenuated, since when the piston isas close to the tuned circuit as its limit of travel permits there willbe a large minimum loss in the amplitude of the picked up signal. Thislarge minimum loss reduces the available power output of the system.

The present invention seeks to provide an improved signal generatorutilizing a piston-in-waveguide attenuator in which the effects ofattenuation reaction is reduced by a method not suffering from thedisadvantages of the methods outlined above.

According to this invention a signal generator comprises a tunedcircuit, a piston-in-waveguide attenuator coupled to said tuned circuit,means for deriving a control voltage dependent upon the position of thepiston in the waveguide of the attenuator, and means for utilizing saidcontrol voltage to provide a compensating change in reactance to counterchange in reactance of the tuned circuit caused by the proximity of saidpiston to the circuit so as to maintain the output frequency of thesignal generator substantially constant.

Preferably said means for deriving a control voltage comprise apotential divider including a potentiometer, the position of the wiperof the potentiometer being determined by the position of the piston inthe waveguide of the attenuator.

Preferably the potentiometer has a logarithmic law.

Preferably switch means are provided to render said means for deriving acontrol voltage inoperative when the piston is greater than apredetermined distance away from the tuned circuit.

Preferably said switch means is controlled by a ramp mounted on saidpiston which engages with a switch control lever.

Preferably said means for utilizing the control voltage include avariable capacity diode, the control voltage being a biassing voltageapplied to said diode so as to vary its capacity.

The invention will now be described with reference to the accompanyingdrawings which is a diagrammatic representation of one embodiment of theinvention.

The FlGURE shows a signal generator having a piston-inwaveguideattenuator.

The tuned circuit of the signal generator includes a variable capacitor1 and a resonant cavity 2. Coupled to the resonant cavity 2 is a lengthof waveguide 3. Mounted within the waveguide 3 is a piston 4 providedwith a pick-up element, shown as a pick-up resistor 5, at the end of thepiston closest to the cavity 2. The resistor is connected to a length ofcoaxial cable which extends through the piston 4 to an output terminal7.

The piston 4 is provided with a rack 8 which engages with the teeth 9 ofa gear wheel 10. It can be seen that when the gear wheel 10 is rotatedin the direction indicated by the arrow the piston 4 will be drivenalong the waveguide 3 and the pick-up resistor will approach closer tothe cavity 2, thereby reducing the attenuation of the output signalproduced at output terminal 7.

input signals to the tuned circuit are applied at input terminal 11,together with a negative d.c. biassing potential which biasses avariable capacity diode 12 also forming part of the tuned circuit of thegenerator and which is connected in series with a capacitor 13 betweenthe input terminal 11 and earth. The potential across the variablecapacity diode 12 determines the capacity of the diode, and thus thefrequency of the tuned circuit. The input signals applied to terminal 11can be used to provide frequency modulation or calibrated incrementalfrequency changes of the output signal from the generator.

As the pick-up resistor 5 is driven towards the cavity 2 the proximityof the piston 4 to the cavity alters the reactive loading of the tunedcircuit, thus altering its resonant frequency. This change in resonantfrequency is compensated for by applying a biassing potential to thejunction of the variable capacity diode 12 and the capacitor 13 suchthat the frequency of the tuned circuit is maintained at a substantiallyconstant value. This biassing voltage is derived from a negative d.c.potential source 14 by means of a resistor 15 and a potentiometer 16connected in series between the potential source 14 and earth. Thecontrol voltage is derived from the wiper of potentiometer 16, which isdirectly connected to the junction point of the resistor 15 and thepotentiometer 16. The control voltage is applied to the junction pointof variable capacity diode l2 and capacitor 13 by cable 17.

Connected across potentiometer 16 is a switch 18. This switch isnonnally closed, but it is arranged to be opened when the piston 4approaches close enough to the cavity 2 to alter its reactive loading. Asuitable means of opening the switch comprises a ramp 19 mounted on thepiston which is positioned to engage with a switch control lever 20 whenthe piston is in a suitable position. As the piston is withdrawn fromthe cavity the switch control lever 20 will disengage from the ramp 19,closing the switch once more.

The wiper of the potentiometer is driven up the potentiometer resistanceelement by means of drive wheel 21, which in turn drives gear wheel 10.Since gear wheel 10 determines the positioning of the piston, and sincethe position of the potentiometer wiper is also determined by the gearwheel 10, via drive wheel 21, the positioning of the potentiometer wiperis directly related to the positioning of the piston.

As the piston 4 approaches the cavity 2 from a large distance the switch18 is initially closed, thus shorting the resistance element of thepotentiometer 16. It can be seen that the cable 17, and thus thejunction of the variable capacity diode 12 and the capacitor 13, will beconnected to earth. Thus the frequency of the tuned circuit dependssolely upon the negative dc. bias applied to the input terminal 1 1.

When the piston 4 is so close to the cavity 2 that it affects thereactive loading of the tuned circuit the ramp 19 engages with theswitch control lever 20, thereby opening the switch 18. The wiper of thepotentiometer 16 is at the end of the potentiometer resistance elementconnected to earth, and so the control voltage applied to cable 16 isstill earth voltage. As the piston is driven closer to the cavity by thegear wheel the drive wheel 21 rotates, driving the wiper ofpotentiometer 16 up the resistance element away from the end connectedto earth potential. Thus a negative control voltage is applied via thecable 17 to the junction point of the variable capacity diode l2 and thecapacitor 13. This control voltage is such that the change of frequencycaused by the variation of the capacity of the variable capacity diodecancels out the change of frequency caused by the proximity of thepiston to the cavity 2. It has been found that the best results areobtained when the potentiometer 16 has a logarithmic law.

As the piston 4 is withdrawn from the cavity 2 the wiper of thepotentiometer is driven back to the end of the resistance elementconnected to earth, and then the ramp l9 disengages the switch controllever 20, thereby closing the switch 18.

I claim:

1. A signal generator comprising a tuned circuit, a piston-inwaveguideattenuator coupled to said tuned circuit, means for deriving a controlvoltage dependent upon the position of the piston in the waveguide ofthe attenuator, and means for utilizing said control voltage to providea compensating change in reactance to counter change in reactance of thetuned circuit caused by the proximity of said piston to the circuit soas to maintain the output frequency of the signal generatorsubstantially constant.

2. A generator as claimed in claim 1 wherein said means for deriving acontrol voltage comprise a potential divider including a potentiometer,the position of the wiper of the potentiometer being determined by theposition of the piston in the waveguide of the attenuator.

3. A generator as claimed in claim 2 wherein the potentiometer has alogarithmic law.

4. A generator as claimed in claim 1 wherein switch means are providedto render said means for deriving a control voltage inoperative when thepiston is greater than a predetermined distance away from the tunedcircuit.

5. A generator as claimed in claim 4 wherein said switch means iscontrolled by a ramp mounted on said piston which engages with a switchcontrol lever.

6. A generator as claimed in claim 1 wherein said means for utilizingthe control voltage include a variable capacity diode, the controlvoltage being a biassing voltage applied to said diode so as to vary itscapacity.

1. A signal generator comprising a tuned circuit, a piston-inwaveguideattenuator coupled to said tuned circuit, means for deriving a controlvoltage dependent upon the position of the piston in the waveguide ofthe attenuator, and means for utilizing said control voltage to providea compensating change in reactance to counter change in reactance of thetuned circuit caused by the proximity of said piston to the circuit soas to maintain the output frequency of the signal generatorsubstantially constant.
 2. A generator as claimed in claim 1 whereinsaid means for deriving a control voltage comprise a potential dividerincluding a potentiometer, the position of the wiper of thepotentiometer being determined by the position of the piston in thewaveguide of the attenuator.
 3. A generator as claimed in claim 2wherein the potentiometer has a logarithmic law.
 4. A generator asclaimed in claim 1 wherein switch means are provided to render saidmeans for deriving a control voltage inoperative when the piston isgreater than a predetermined distance away from the tuned circuit.
 5. Agenerator as claimed in claim 4 wherein said switch means is controlledby a ramp mounted on said piston which engages with a switch controllever.
 6. A generator as claimed in claim 1 wherein said means forutilizing the control voltage include a variable capacity diode, thecontrol voltage being a biassing voltage applied to said diode so as tovary its capacity.